The development of plastic blood collection bags facilitated the separation of donated whole blood into its various components, e.g., platelet concentrate (hereinafter "PC"), packed red cells (hereinafter "PRC"), and plasma, thereby making platelet concentrates available as a transfusion product. The separation of a single unit of donated whole blood, about 450 milliliter in USA practice, into its components is typically accomplished by use of differential sedimentation.
A typical procedure used in the U.S., the citrate-phosphate-dextrose-adenine (CPDA-1) system, utilizes a series of steps to separate donated blood into three components, each component having substantial therapeutic and monetary value. The procedure typically utilizes a blood collection bag which is integrally attached via tubing to at least one, and preferably two or more, satellite bags. Whole blood may be thus collected and processed as follows:
(1) The donated whole blood is collected from the donor's vein directly into the blood collection bag which contains the nutrient and anti-coagulant containing CPDA-1.
(2) The blood collection bag is centrifuged together with its satellite bags, thereby concentrating the red cells as packed red cells (hereinafter PRC) in the lower portion of the blood collection bag and leaving in the upper portion of the bag a suspension of platelets in clear plasma, which is known as platelet-rich plasma (PRP).
(3) The blood collection bag is transferred, with care not to disturb the interface between the supernatant PRP layer and the sedimented PRC layer, into a device known as a "plasma extractor" which comprises an opaque back plate and a transparent front plate; the two plates are hinged together at their lower ends and spring biased toward each other such that a pressure of about 90 millimeters of mercury is developed within the bag.
With the blood collection bag positioned between the two plates, a valve or seal in the tubing is opened allowing the supernatant PRP to flow into a first satellite bag. As the PRP flows out of the blood collection bag, the interface with the PRC rises. The operator closely observes the position of the interface as it rises and clamps off the connecting tube when in his judgment as much PRP has been transferred as is possible, consistent with allowing no red cells to enter the first satellite bag. This is a time consuming operation during which the operator must visually monitor the bag and judiciously and arbitrarily ascertain when to shutoff the connecting tube. The blood collection bag, now containing only PRC, may be detached and stored at 4.degree. C. until required for transfusion into a patient, or a valve or seal in the flexible tubing may be opened so that the PRC may be transferred to a satellite bag using either the pressure generated by the plasma extractor apparatus, or by placing the blood collection apparatus in a pressure cuff, or by elevation to obtain gravity flow.
(4) The PRP-containing satellite bag, together with another satellite bag, is then removed from the extractor and centrifuged at an elevated G force with the time and speed adjusted so as to concentrate the platelets into the lower portion of the PRP bag. When centrifugation is complete, the PRP bag contains sedimented platelets in its lower portion and clear plasma in its upper portion.
(5) The PRP bag is then placed in the plasma extractor, and most of the clear plasma is expressed into the other satellite bag, leaving the PRP bag containing only sedimented platelets in about 50 ml of plasma; in a subsequent step, this platelet composition is dispersed to make PC. The PRP bag, now containing a PC product, is then detached and stored for up to five days at 20.degree.-22.degree. C., until needed for a transfusion of platelets. For use with adult patients, the platelets from 6-10 donors are, when required, pooled into a single platelet transfusion.
(6) The plasma in the other satellite bag may itself be transfused into a patient, or it may be separated by complex processes into a variety of valuable products.
Commonly used systems other than CPDA-1 include Adsol, Nutricell, and SAG-M. In these latter systems, the collection bag contains only anticoagulant, and the nutrient solution may be preplaced in a satellite bag. This nutrient solution is transferred into the PRC after the PRP has been separated from the PRC, thereby achieving a higher yield of plasma and longer storage life for the PRC.
With the passage of time and accumulation of research and clinical data, transfusion practices have changed greatly. One aspect of current practice is that whole blood is rarely administered; rather, patients needing red blood cells are given packed red cells, patients needing platelets are given platelet concentrate, and patients needing plasma are given plasma.
For this reason, the separation of blood into components has substantial therapeutic and monetary value. This is nowhere more evident than in treating the increased damage to a patient's immune system caused by the higher doses and stronger drugs now used during chemotherapy for cancer patients. These more aggressive chemotherapy protocols are directly implicated in the reduction of the platelet content of the blood to abnormally low levels; associated internal and external bleeding additionally requires more frequent transfusions of PC, and this has caused platelets to be in short supply and has put pressure on blood banks to increase platelet yield per unit of blood.
Blood bank personnel have responded to the increased need for blood components by attempting to increase PC yield in a variety of ways, including attempting to express mere PRP prior to stopping flow from the blood collection bag. This has often proved to be counterproductive in that the PRP, and the PC subsequently extracted from it, are frequently contaminated by red cells, giving a pink or red color to the normally light yellow PC. The presence of red cells in PC is so highly undesirable that pink or red PC is frequently discarded, or subjected to recentrifugation, both of which increase operating costs.
The method and apparatus of the present invention alleviate the above-described problems and, in addition, provide a higher yield of superior quality PC.
In addition to the three above-listed components, whole blood contains white blood cells (known collectively as leucocytes) of various types, of which the most important are granulocytes and lymphocytes. White blood cells provide protection against bacterial and viral infection.
The transfusion of blood components which have not been leucocyte-depleted is not without risk to the patient receiving the transfusion. Chills, fever, and allergic reactions may occur in patients receiving acute as well as chronic platelet therapy. Repeated platelet transfusions frequently lead to alloimmunization against HLA antigens, as well as platelet specific antigens. This, in turn, decreases responsiveness to platelet transfusion. Leucocytes contaminating platelet concentrates, including granulocytes and lymphocytes, are associated with both febrile reactions and alloimmunization, leading to platelet transfusion refractoriness. Another life-threatening phenomenon affecting heavily immunosuppressed patients is Graft Versus Host Disease. In this clinical syndrome, donor lymphocytes transfused with the platelet preparations can launch an immunological reaction against the transfusion recipient with pathological consequences. Some of these risks are detailed in U.S. Pat. No. 4,923,620 and in U.S. Pat. 4,880,548.
In the above described centrifugal method for separating blood into the three basic fractions, the leucocytes are present in substantial quantities in both the packed red cells and platelet-rich plasma fractions. It is now generally accepted that it would be highly desirable to reduce the leucocyte concentration of these blood components to as low a level as possible. While there is no firm criterion, it is generally accepted that many of the undesirable effects of transfusion would be reduced if the leucocyte contempt were reduced by a factor of about 100 or more prior to administration to the patient. This approximates reducing the average total content of leucocytes in a single unit of PRC or PRP to less than about 1.times.10.sup.7, and in a unit of PRP or PC to less than about 1.times.10.sup.6.
Growing evidence suggests that the use of leucocyte depleted platelet concentrates decreases the incidence of febrile reactions and platelet refractoriness. Leucocyte depleted blood components are also believed to have a role in reducing the potential for Graft Versus Host Disease. Leucocyte depletion of platelet preparations is also believed to diminish, but not to completely prevent, the transmission of leucocyte associated viruses such as HIV-1 and CMV.
Platelet preparations contain varying amounts of leucocytes. The level of leucocyte contamination in unfiltered conventional platelet preparations of 6 to 10 pooled units is generally at a level of about 5.times.10.sup.8 or greater. Platelet concentrates prepared by the differential centrifugation of blood components will have varying levels of leucocyte contamination related to the time and to the magnitude of the force developed during centrifugation. It has been demonstrated that leucocyte removal efficiencies of 81 to 85% are sufficient to reduce the incidence of febrile reactions to platelet transfusions. Several other recent studies report a reduction in alloimmunization and platelet refractoriness at levels of leucocyte contamination &lt;1.times.10.sup.7 per unit. For a single unit of PC, the goal is to reduce the number of leucocytes from about 7.times.10.sup.7 leucocytes (average leucocyte contamination level under current practice) to less than about 1.times.10.sup.6 leucocytes. The existing studies therefore suggest the desirability of at least a two log (99%) reduction of leucocyte contamination. More recent studies suggest that a three log (99.9%) or even a four log (99.99%) reduction would be significantly more beneficial.
An additional desirable criterion is to restrict platelet loss to about 15% or less of the original platelet concentration. Platelets are notorious for being "sticky", an expression reflecting the tendency of platelets suspended in blood plasma to adhere to any non-physiological surface to which they are exposed. Under many circumstances, they also adhere strongly to each other.
In any system which depends upon filtration to remove leucocytes from a platelet suspension, there will be substantial contact between platelets and the internal surfaces of the filter assembly. The filter assembly must be such that the platelets have minimal adhesion to, and are not significantly adversely affected by contact with, the filter assembly's internal surfaces.
U.S. Pat. No. 4,880,548 provides a convenient and very effective means for leuco-depleting PC. PC is passed through a fibrous porous medium which permits recovery of 90% or more of the platelets, which pass through the medium, while retaining within the medium more than 99.9% of the incident leucocytes. This system is currently in widespread use at bedside in hospitals, but, unlike the device of this invention, it is not as well suited for use in blood banks during the processing of donated whole blood. The unsuitability stems primarily from additional storage constraints associated with PC and the methods of administering PC. For example, platelets in PC are typically suspended in a total volume of only about 40 to 60 ml of plasma. Contrasted with this, the platelets which are processed in accordance with this invention are typically derived from a single unit of whole blood and are suspended as PRP in about 180 to 240 ml of plasma.
Further, the platelets in PC have been subjected, during two centrifugation steps, to severe conditions and may not as readily disperse. It has been suggested that the high forces to which the platelets are subjected as they reach the bottom of the bag during sedimentation, promote increased aggregation by particle-to-particle adhesion.
For these and perhaps other reasons, platelets in PC show a much higher tendency to be retained within the filter during leucocyte depletion compared with platelets in PRP. Accordingly, a much better recovery is obtained when platelets are leucocyte-depleted in the form of PRP, compared with PC; for example, while optimal recovery from PC is about 90 to 95%, recovery from PRP can exceed 99%.
Also, as a consequence of the concentration differences and possibly as a consequence of the lower degree of aggregation in PRP, the preferred critical wetting surface tension (CWST) range when filtering PRP is broader than that for PC.
Devices which have previously been developed in attempts to meet the above-noted objectives have been based on the use of packed fibers, and have generally been referred to as filters. However, it would appear that processes utilizing filtration based on separation by size cannot succeed for two reasons. First, leucocytes can be larger than about 15 .mu.m (e.g., granulocytes and macrocytes) to as small as 5 to 7 .mu.m (e.g., lymphocytes). Together, granulocytes and lymphocytes represent the major proportion of all of the leucocytes in normal blood. Red blood cells are about 7 .mu.m in diameter, i.e., they are about the same size as lymphocytes, one of the two major classes of leucocytes which must be removed. Secondly, all of these cells deform so that the are able to pass through much smaller openings than their normal size. Accordingly, it has been widely accepted that removal of leucocytes is accomplished mainly by adsorption on the internal surfaces of porous media, rather than by filtration.
The separation of the various blood components using centrifugation is attended by a number of problems. First, in the separation of platelet-rich plasma from PRC, e.g., step 3 above, it is difficult to efficiently obtain the maximum yield of platelets while preventing red cells from entering the plasma. Secondly, when PRP is expressed, it is difficult to efficiently recover the more desirable younger platelets located near or in the PRC/PRP interface.